[ThinLTO] Always emit a summary when compiling in ThinLTO mode

Summary:
Emit an empty summary section, instead of no summary section, when
there are no global variables in the index. This ensures that LTO
will treat these files as ThinLTO inputs, instead of as regular
LTO inputs.

In addition to not being what the user likely intended when
compiling with -flto=thin, the current behavior is problematic for
distributed build systems that expect to get ThinLTO index and imports
files back for each input compiled with -flto=thin. Combining into
a single regular LTO module also reduces the backend parallelism.
And in the case where the index was suppressed due to uses in
inline assembly, combining into a single LTO module could provoke
renaming of duplicates that we were trying to prevent by suppressing
the index.

This change required a couple of fixes to handle the empty summary
section.

Reviewers: mehdi_amini

Subscribers: mehdi_amini, llvm-commits, pcc

Differential Revision: https://reviews.llvm.org/D24779

llvm-svn: 282037

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index 0d47cd49a17..017a45ac7ba 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -713,6 +713,16 @@ Error LTO::runThinLTO(AddOutputFn AddOutput, bool HasRegularLTO) {
       ModuleToDefinedGVSummaries(ThinLTO.ModuleMap.size());
   ThinLTO.CombinedIndex.collectDefinedGVSummariesPerModule(
       ModuleToDefinedGVSummaries);
+  // Create entries for any modules that didn't have any GV summaries
+  // (either they didn't have any GVs to start with, or we suppressed
+  // generation of the summaries because they e.g. had inline assembly
+  // uses that couldn't be promoted/renamed on export). This is so
+  // InProcessThinBackend::start can still launch a backend thread, which
+  // is passed the map of summaries for the module, without any special
+  // handling for this case.
+  for (auto &Mod : ThinLTO.ModuleMap)
+    if (!ModuleToDefinedGVSummaries.count(Mod.first))
+      ModuleToDefinedGVSummaries.try_emplace(Mod.first);
 
   StringMap<FunctionImporter::ImportMapTy> ImportLists(
       ThinLTO.ModuleMap.size());